1. Field of the Invention
This invention relates to the control of the liquid level in a sump, wet well, reservoir, drainage canal, bilge of a vessel, or the like. In particular, this invention relates to a solid state electronic control system to control duplex motor starters designed to power pumps.
2. General Background
In the past, duplex pump controllers have been utilized for various types of circuits, gauges, and switches to form a liquid level controller. Each of the means used has its own disadvantages.
Conventional relay systems, the most common type of duplex pump control that is in use, present two main disadvantages. First, if the system utilizes line voltage on mercury floats which is in a wet well, the system will not be safe. If low voltage is used, the system will be safe if the proper relays in a low system voltage is used. The disadvantages are that low voltage transformers, isolation relays for each float switch, and a current limiting device to maintain safe currents and voltages are required. This equipment takes up extra space, uses extra power, and creates extra heat.
Hybrid circuits, combinations of solid state and conventional relay circuits, have been used but are prone to failure due to the combination and often require float isolation relays to maintain safe float switch voltages and currents. These circuits also increase dissipation of power and require extra space.
Some duplex pump controllers have utilized complex solid state circuitry wherein a pressure sensitive strain gauge is coupled to solid state logic circuitry. The resulting system is safe but has disadvantages in that it is very complex, requires an air compressor to operate the strain gauge and numerous other components, is very costly, and requires trained technicians to make repairs due to its complexity.
Many systems use a pressure switch gauge connected to a conventional relay system. Although they are commonly used and safe, there are disadvantages. An air compressor, switch gauge, and relay logic are required and because of its mechanical nature, the system is prone to failure. This system is also costly in initial investment and maintenance after approximately two years in operation.
The oldest method, using a float ball and rod, is safe and reliable but does not afford the needed true control of the pumping sequence in a duplex pump system.